Communication system



March 31, 1942. L `Y LACY coMMNIcATIoN SYSTEM Filed Oct. 24, 1940 2 Sheets-Sheet 1 NETWORK LOAD ATTORNEY March 31, 1942. L. Y. LACY 2,278,177

COMMUNICATION SYSTEM Filed Oct. 24, 1940 2 Sheets-Sheet 2 AAAA VVV

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BVMW

ATTORNEY Patented Mar. 31, 1942 COMMUNICATION SYSTEM Lester Y. Lacy, Madison, N. 1.,' assigner to Bell Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application October 24, 1940, Serial No. 362,544

6 Claims.

This invention relates to communication systems, and more particularly Ato an arrangement for minimizing the effect of interfering currents introduced into such systems.

When signals or other useful energy is transmitted over a line comprising a plurality of conductors, one of which has each of its terminals connected to a local ground, whereas the other conductor is isolated from ground except at its terminals where it is connected to ground via terminating networks, both conductors, being associated via the local terminal connections with a ground return, will constitute a complete circuit. If sources external to the communication system produce a difference of potential between the locally applied ground connections, extraneous currents of considerable magnitude and of the frequency of the external sources may be caused to iiow through such circuit. When the extraneous currents lie in a portion of the frequency rangev occupied 4by the useful or signal energy, they may seriously interfere with the efficient transmission of the latter energy.

In one typical example of the above type of lcommunication system, the signal or useful energy is transmitted between sending and receiving stations over a path comprising a coaxial conductor having the terminals of its outer conductor grounded. As is well known, such system may be expeditiously adapted for the transmission of signal currents extending over a wide frequency range, such as television or other signal currents, which extends, for example, from an upper limiting frequency of one or more million cycles per second down to a few cycles per second. The coaxial conductor embodied in such systems is enclosed in, but insulated from, a grounded sheath.

The outer conductor of a coaxial conductor operates to shield effectively the signal transmission path against high frequency currents iiowing in the outer conductor, but for current in the low frequency portion of the spectrum, the eiliciency of the latter conductor as a shielding element is relatively low. Consequently, earth po-y tentials or voltages induced in the coaxial conductor from'external sources may introduce low frequency circulating current in the signal transmission path to impair seriously the efficient transmission of signals. The signal transmission path is protected against the introduction of noise currents through the sheath, as the latter is effective as a shielding element at the high frequency end of the range, and the insulation, between the sheath and the concentric conductor,

serves to isolate the latter at the low frequency end of the range.

The present invention will be hereinafter described as applied to a communication system in which television image currents having a frequency band of the order of one or more million cycles per second are transmitted over a coaxial conductor transmission line.

Accordingly, the present invention contemplates for use in such system an arrangement to minimize the effect of interfering noise currents introduced into the signaling path of a coaxial conductor line while at the same time, to avoid substantially any disturbance of the transmission of the signal currents, or, in other words, to increase substantially the signal-to-noise ratio.

A general object of the invention is to discriminate 'favorably with respect to signaling currents and unfavorably with respect to noise currents. c

Another object is to produce equal noise voltages at the output terminals of the inner and outer conductors of a coaxial conductor line.

A further object is to eliminate at the output terminal of a coaxial conductor line the noise voltages without materially interfering with the signal voltages. ,A

In accordance with a4 preferred embodiment,

' the invention comprises a coaxial conductor line lhaving inner and outer conductors terminated at their input and output terminals with suitable impedance networks and ground connections, and arranged such that both the inner and outer conductors and the input and output terminating impedance networks constitute the arms of a bridge network,` with the output terminating network between the output terminals of the inner and outer conductors forming one diagonal, and ground connections at the input and output terminals applied to the opposite diagonal. Noise currents introduced in the inner and outer conductors ilow in circuits including the latter,v the terminating networks and the r ground connections. A balanced condition of this bridge network serves to produce equal noise voltages to .ground on the inner and outer con- A network coupling the output terminating network to a load discriminates between the noise and signal voltages such that the noise voltages are eliminated from the load and the signal voltages are supplied thereto.

The invention will be readily understood from the following description taken together with the accompanying drawings, in which:

Fig. 1A is a schematic representation of a coaxial transmission system operating without the present invention;

Fig. lB is a bridge arrangement of Fig. 1A;

Fig. 2A is a schematic illustration of a simplified form of the invention;

Fig. 2B is a bridge form of Fig. 2A;

Fig'. 3A is a schematic delineation of a coaxial transmission system embodying the form of the invention illustrated in Figs. 2A and 2B;

Fig. 3B is a bridge arrangement of Fig. 3A; and

Figs. 4A, 4B, 5A, 5B, 6A and 6B are schematic illustrations` of other simplified forms of the invention.

Referring to Fig. 1A, a coaxial conductor line L utilized for signaling transmission comprises an inner conductor Z1 and an outer conductor Zo. both of which are enclosed in, but insulated from. a sheath Z.. At the left-hand or input end the inner and outer conductors are terminated in a network Zx which -is also connected to the sheath Z. applied to ground connection Gi. At the right-hand or output end Va ground connection G is common to both the outer conductor Zo and sheath Z. while a terminating network Zy is applied across the inner, and outer conductors. It is to be noted that the terminating network Z, is also connected to the ground connection G0. It is assumed that the line L extends through a region in which a noise voltage source En. external to a signal system and effective through the ground connections G0 and Gi, causes noise current to flow from ground con-4 nection G1, which has a different potential than ground connection G0, to the ground connection Go through several paths embodying the .sheath and inner and outer conductors, and the input and output terminating networks.

As pointed out above this noise voltage En may introduce noise currents into the signaling transmission path. Hence, noise current may flow in a path comprising ground connection G1, input terminating network ZX, inner conductor Zi, output terminating network Zy and ground connection G0. The noise current iowing in the output terminating network Zy will produce unequal noise voltages at the opposite terminals there- Q of. As the output terminating network Zy is also commonto a signal load, not shown, such noise voltages may tend to interfere with the transmission ci' signals. Fig. 1B represents the transmission line of Fig. 1A arranged in the form of a bridge, with the arms and diagonale embodying elements corresponding to the respective elements of the line. Thus, in Figs. 1A and 1B unequal noise voltages may exist at the terminals E and A.

However, if an impedance were interposed between the output terminals A and B, Figs. 1A and 1B, it is obvious that the iiow of noise current through the output terminating network Z, would tend to decrease, depending on the value of such impedance, and would be substantially zero as the value of such impedance apf preached infinity. This condition is usually attainable in practice only at low frequencies where the shielding effect of the outer conductor is limited,r as the distributed capacity between the outer conductor and sheath of the coaxial conductor permits a flow of some noise current. As the flow of noise current increases with fre- 'a receiving station RS connected to the outpul amai?? quency, reduction of the former tends to become somewhat less at the higher frequencies.

In accordance with the invention, Fig. 2A showsI in the output terminal of the outer conductor Z0 an impedance Zz disposed between the points A and B and corresponding to the impedance mentioned in the next preceding paragraph, and in addition an impedance Z applied in the output terminal between the inner conductor Zi and ground connection G0. The are rangement of Fig. 2A which also includes the portion shown to the left of the line YY in Fig. 1A may be represented as a bridge as illustrated in Fig. 2B. For minimum residual noise energy appearing across the output terminating impedance Zy, the proper relationship of the various arms of the bridge is expressed as follows:

in which Zo=impedance of the outer conductor of the conductor line L.

Zi=impedance of the inner conductor of the concentric conductor line L.

Zx=input terminating network.

Zz=output termination of the outer conductor.

Z :output bridge termination of the inner conductor.

Zy=termination applied across output terminals of inner and outer conductors.

Zs=impedance of sheath.

En=voltageof external or noise source.

Ey=noise voltage across output impedance Zy.

The value of Zz is not critical from a noise standpoint, but once it is selected, it obviously requires a definite value of Z as indicated in the above equation.

In the event that two concentric conductors are embodied within the same sheath, cross-talk between two such conductors is dependent on the value of Zz, and, therefore, this value is initially selected to provide minimum cross-talk. Thereafter, the value of Z may be adjusted to eifect maximum noise reduction n the output terminating network Zy.

Fig. 3A illustrates the arrangements of Figs. 2A and 2B embodied in a signaling transmission system which comprises a transmitting station TS applied to the input terminating network ZX, and

terminating network Zy. At the receiving station a voltage discriminating network N connects the output terminating network lily to a suitable load, and functions in a manner that will be hereinafter explained. It is to be understood that the portion included between the lines X-X and Y-Y of Fig. 1A is to be included between the similarly identified lines of Fig. 3A, and, further, that the arrangement of Fig. 3A may be represented by the bridge illustrated in Fig. 3B.

Station TS may include a studio in which a suitable transmitter serves to scan a field of view or subject of any type, including a motion picture film, and to control the production of an image current extending over a frequency band having an upper limiting frequency of one or more million cycles per second down to a few cycles per second. StationRS may be a distributing point from which the image current may be supplied over lines in its normal frequency range, or it may be a broadcasting station from which the image current is transmitted as a modulation of a carrier wave over a line or link to a remote television subscribers station, either alone or in association with accompanying sound current; or it may be any station at which television image current is received in its normal frequency range,

whence the image current is redistributed to a.

.noise voltages will exist across the respective upper and lower terminals E and A of the output terminating network Zy with regard to the output ground connection G0, Figs. 3A and 3B.

The coupling network N serves to discriminate between the signal voltage appearing across the output terminating network Zy and the equal noise voltages appearing on the terminals E and A of theoutput terminating impedance Zy such that the equal noise voltages are eliminated and the signal voltages are passed to the load. A-

suitable coupling network for effecting such` volttage discrimination is disclosed in the copending applica'ion of S. Doba, Jr., Serial No. 303,778, filed November 10, 1939, now Patent No. 2,226,238, vDecember 24, 1940. It is to be understood that a well-balanced transformer would also serve to provide the ksame discrimination betwen such noise and signal voltages.

The arrangements of Figs. 1A, 1B, 2A, 2B, 3A and 3B are described with respect to the introduction of noise currents in the signalingpath of the line from an external source through earth potentials eiiective via ground connections Gi and G0. In addition the invention is equally applicable to a condition in which the noise lvoltages are magnetically induced in the signaling path of the line. This is represented in Figs. 4A and 4B both of which are otherwise identical with the respective Figs? 2A and 2B.

Also, the invention is similarly applicable to a coaxial conductor line which is not enclosed within a sheath, such, for example, as a line mounted on poles. Such application is illustrated in Figs. 5A and 5B both of which are otherwise the same as the respective Figs. 2A and 2B. Further, the

' invention is expeditiously serviceable in the case in which the noise voltages are introduced in the signal path via magnetic induction as illustrated in Figs. 6A and 6B, which are otherwise the same as Figs. 4A and 4B.

It is to be understood that the arrangements illustrated in' Figs. 4A, 5A and 6A may be embodied in a signaling transmission system in the manner illustrated in Fig. 3A b y substituting between the lines X-X and Y--Y of the latter the portions between the lines X-X and Y-Y of the former, respectively.

While the inventionv has been described with' which the noise currents are introduced through one of the conductors embodied therein.

What is claimed is:

1. In combination, a coaxial conductor line comprising inner and outer conductors and having input and output terminals at opposite ends, each end of said line including a local ground connection, means to apply input waves to said input terminals for transmission over said line to the opposite end and a load at said latter end connected to said output terminals, and means to protect said load against disturbing voltages existing between said ground connections comprising impedances connected to said load and said terminals and forming with said conductors and said load a vbridge configuration in which said load and the effective impedance between said ground connections appear in respectively conjugate diagonals.

with other types of transmission systems into 2. In' combination, a coaxial conductor line comprisinginner and outer conductors and subject to the introduction of noise currents therein from a noise source external of said line, an impedance network applied across the input terminals of both said conductors, an impedance network applied across the output terminals of both said conductors, a ground connection common to each of said input and output terminating networks and said outer conductor so that the noise currents may flow in circuits including both said input and output ground connections, both said inner and outer conductors, and both said input and output terminating networks and thereby to produce unequal noise voltages at the opposite terminals of said output terminating network, and means to equalize the noise voltages produced at the opposite terminals of said output terminating network, said voltage equalizing means comprising an-impedance network applied between the output terminal of said inner conductor and said output ground connection, an impedance network disposed between the output terminal of said outer conductor and said output ground connection, and both said latter networks together with both said conductors and said input and output terminating networks applied between both said conductors constituting a bridge network comprising in series a first arm including said input terminating network and said inner conductor, a second arm including said impedance network connected between the output terminal ofsaid inner conductor and said output l ground connection, a third arm including said impedance network. connected between the output terminal of said outer conductor and said output ground connection, a fourth arm including said outer conductor, one diagonal including said output terminating network applied between the output terminals of both said conductors and the opposite diagonal connected to both said input and output ground connections.

3. The combination according to claim 2 in which the noise currents vare introduced in the circuit including both. said input and output ground connections, said input terminating network, said inner conductor and said output terminating network through both said input and output ground connections.

4. The combination according to claim 2 in which the noise currents are magnetically induced in the circuit includingboth said input and output gro nd' connections, said input terminating networ said inner conductor and said output terminating network.

5. A communication system comprising a line including a coaxial conductor comprising inner and outer conductors over which signaling current may be transmitted, said line extending through an area. including a source of noise current in the same frequency range as at least a part of the signaling current and which noise current may be introduced in said inner conductor, a network to terminate the input termi nais of both said conductors, signaling means applied to said input terminating network,. a

voltages that tend to interfere with the sig naling voltages which also appears across the opposite terminals of said output terminating network and are applied to said load, and means to eliminate the effect of the unequal noise voltages from said output terminating network and said load and to apply the signaling voltages to said load, said last-mentioned means comprising an impedance network connecting the output terminal of said outer conductor to said output ground connection, an impedance network connecting the output terminal of said inner conductor to said output groundconnection and a voltage discriminating network applied to the opposite terminals of said outputl terminating network and interposed between said latter network and said load.

6. The communication system in accordance with claim 5 in which both said inner and outer conductors, both said input and output terminating networks, both said inner and outer conductor output terminating impedance networks and both said input and output ground connections constitute a bridge network comprising a iirst arm including in series said input terminating network and said inner conductor, a second arm including said impedance network connected between the output terminal of said inner conductor and said output ground connection, a

`third arm including said impedance network applied between theoutput terminal of said outer conductor and said output ground connection, `a fourth arm including said outer conductor, one diagonal including insequence said output terminating network, Asaid voltage discriminating means and said load, and the opposite diagonal connected to said input and output ground connections.

LESTER Y. LACY. 

